1. Field of the Invention
The present invention relates to an optical pickup apparatus for recording or reproducing information on or from an optical recording medium.
2. Description of the Related Art
High output power semiconductor lasers are used as light sources of optical pickup apparatuses, in particular, ones capable of writing information to an optical recording medium. In such a high output power semiconductor laser, the reflectance of each of anti-reflecting coatings of the front and rear faces of a laser chip of the semiconductor laser is changed by changing its thickness. Therefore, the quantities of light beams that are emitted from the front and rear faces of the laser chip are different from each other. This makes it impossible to detect light that is emitted from the rear face by providing a light detecting device inside a semiconductor laser package and control the power of light that is emitted from the front face on the basis of a detection result.
For the above reason, in an optical pickup apparatus that is equipped with a semiconductor laser having a high output power, it is necessary to monitor the power of light that is emitted from the front face of a laser chip by providing a light detecting device for monitoring the optical output power of the semiconductor laser outside a semiconductor laser package and inside the optical pickup apparatus.
Among the conventional optical pickup apparatuses in which a light detecting device (front monitor) for monitoring the power of light emitted from the front face of a laser chip the semiconductor laser is provided outside a semiconductor laser package and inside the optical pickup apparatus is the following (e.g., refer to Japanese Unexamined Patent Publication JP-A 8-102080 (1996)).
FIG. 33 shows an example of optical output power monitoring of a conventional optical pickup apparatus 1 that is equipped with two light sources having different wavelengths.
The conventional optical pickup apparatus 1 is equipped with a first light source 2 and a second light source 3 that emit light beams having different wavelengths. A light beam emitted from the first light source 2 is converted into a generally parallel beam in passing through a first collimator lens 4, reflected by a beam splitter 5, reflected by an upward-directing mirror 6 so as to enter an objective lens 7, and focused on the information recording surface of an optical recording medium 8 by the objective lens 7. A light beam reflected by the optical recording medium 8 again passes through the objective lens 7, is reflected by the upward-directing mirror 6, passes through the beam splitter 5 and a half-mirror 9, and is detected by a photodetector 11 via a spot lens 10. A light beams emitted from the second light source 3 is converted into a generally parallel beam in passing through a second collimator lens 12, is reflected by the half-mirror 9, passes through the beam splitter 5, is reflected by the upward-directing mirror 6 so as to enter the objective lens 7, and is focused on the information recording surface of the optical recording medium 8 by the objective lens 7. A light beam reflected by the optical recording medium 8 travels along the same path as the above-described light beam emitted from the first light source does, and is detected by the photodetector 11.
A first front monitor 13 for monitoring the optical output power of the first light source 2 is disposed on the opposite side of the beam splitter 5 to the first light source 2, and detects a light beam that is emitted from the first light source 2 and passes through the first collimator lens 4 and the beam splitter 5. The first front monitor 13 outputs a detection result to a control means 15. The control means 15 controls the power of the light beam that is emitted from the first light source 2 is accordance with the detection output of the first front monitor 13.
On the other hand, a second front monitor 14 for monitoring the optical output power of the second light source 3 is disposed on the opposite side of the half-mirror 9 to the second light source 3, and detects a light beam that is emitted from the second light source 3 and passes through the second collimator lens 12 and the half-mirror 9. The second front monitor 14 outputs a detection result to the control means 15. The control means 15 controls the power of the light beam that is emitted from the second light source 3 is accordance with the detection output of the second front monitor 14.
In the conventional optical pickup apparatus 1 having the above configuration, the first front monitor 13 and the second front monitor 14 are disposed on the opposite side of the beam splitter 5 and the half-mirror 9 to the first light source 2 and the second light sources 3, which raises a problem that the apparatus 1 requires a large installation capacity and has a large size.
FIG. 34 shows an example of optical output power monitoring of another conventional optical pickup apparatus 20. The conventional optical pickup apparatus 20 is similar to the above conventional optical pickup apparatus 1, and hence components of the optical pickup apparatus 20 having corresponding components in the optical pickup apparatus 1 are given the same reference numerals as the latter. In the optical pickup apparatus 20, a hologram laser is used in place of the beam splitter 5 and the hologram laser 5 separates two light beams having different wavelengths from each other.
In another conventional optical pickup apparatus 20, a single front monitor 13 is provided and monitors the optical output powers of both of the first light source 2 and the second light source 3. A light beam emitted from the first light source 2 passes through the hologram laser 5 and is detected by the first front monitor 13. A light beam emitted from the second light source 3 is reflected by the hologram laser 5 and detected by the first front monitor 13. The first front monitor 13 outputs a detection result to the control means 15. The control means 15 controls the optical output power of the first light source 2 and the second light source 3 in accordance with the detection output of the first front monitor 13.
Although another conventional optical pickup apparatus 20 is smaller in the number of parts and hence is smaller in the size of the apparatus than the conventional optical pickup apparatus 1, the former has a problem that the accuracy of the optical power control is lower than in the latter. In general, the ratio between the quantity of transmission light and the quantity of reflection light in the hologram laser 5 is set so that the quantity of light used for recording or reproduction and that used for the optical output power monitoring become about 90% and about 10%, respectively. The quantity of light used for the optical output power monitoring, that is, the quantity of light that is emitted from the first light source 2 and transmits the hologram laser 5 or the quantity of light that is emitted from the second light source 3 and reflected by the hologram laser 5, varies by 10% to 20% depending on the characteristics of the hologram laser 5. It is virtually impossible to realize a beam splitter having such a light separation characteristic as not to cause such a light quantity variation, resulting in a problem that it is difficult to control, with high accuracy, the output power of light that is, emitted from the light source 2 or 3 in the above-described manner.
A semiconductor laser that is one kind of light source provided in an optical pickup apparatus outputs laser light, which is used for recording or reproduction of information on or from an optical recording medium. A semiconductor laser chip that is provided in the semiconductor laser and emits light changes in output power due to a variation in the ambient temperature, a variation with age, etc. The output power of the semiconductor laser chip is kept constant by controlling its drive current with an automatic power control (APC) circuit. Among the methods for keeping the output power of a semiconductor laser chip with the APC circuit are a rear monitoring method and a front monitoring method.
In the rear monitoring method, slight light that is emitted from the semiconductor laser chip in a direction (hereinafter referred to as “rear direction”) that is opposite to a direction in which light is emitted from the semiconductor laser chip to reach an optical recording medium is used for an output power control. Light that is emitted in the rear direction is detected by a power control light detecting means that is provided in the semiconductor laser, and the APC circuit controls the output power of the semiconductor laser to a constant value by supplying it with a signal for controlling a drive current of the semiconductor laser chip in accordance with a detection output of the power control light detecting means.
However, the quantity of light emitted in the rear direction is smaller than that of light emitted in such a direction (hereinafter referred to as “front direction”) as to reach the optical recording medium for recording or reproduction of information on or from the optical recording medium, and is insufficient to control the output power of the semiconductor laser chip.
The ratio of the quantity of light emitted in the rear direction to that of light emitted in the front direction is not necessarily stable. As a result, the rear monitoring method has a problem that the optical output power of the semiconductor laser cannot be controlled with high accuracy.
On the other hand, in the front monitoring method, light that is emitted in the front direction is detected by a monitoring photodetector and the APC circuit controls the output power of the semiconductor laser to a constant value by controlling its drive current in accordance with a detection output of the photodetector. Since, as described above, the quantity of light emitted in the front direction is larger than in the rear direction, to increase the accuracy of the output power control, in general, the front monitoring method is employed dominantly.
FIG. 35 is a simplified system diagram showing the configuration of a still another conventional optical pickup apparatus 31. Light that is emitted from a semiconductor laser chip 33 of a semiconductor laser 32 is converted into parallel light by a collimator lens 34 and enters a beam splitter 35. The beam splitter 35 has, at a prism junction surface 36, a reflection film that transmits 95% of incident light and reflects 5% of it. Therefore, most of incident light is transmitted toward an optical recording medium 40 and the remaining light is reflected toward a power control light detecting means 37.
Light that has passed through the beam splitter 35 is polarized by a quarter-wave plate 38, bent by 90° toward the optical recording medium 40 by an upward-directing mirror 39, and focused by an objective lens 41, whereby a light spot having a prescribed size is formed on the information recording surface of the optical recording medium 40 and information recording or reproduction is performed. Light that has been reflected by the optical recording medium 40 passes through the objective lens 41, bent by 90° by the upward-directing mirror 39, polarized by the quarter-wave plate 38, and input to the beam splitter 35. The beam splitter 35 reflects, 100%, toward a light detecting means 42, the light reflected from the optical recording medium 40. Resulting reflection light is focused by a focusing lens 43, given astigmatism by a cylindrical lens 44, and detected by the light detecting means 42, whereby information on the optical recording medium 40 is read.
Light that has been reflected by the beam splitter 35 toward the power control light detecting means 37 is condensed by a condenser lens 45 and detected by the power control light detecting means 37. An APC circuit 46 controls the output power of the semiconductor laser 32 to a constant value by supplying it with a signal for controlling a drive current of the semiconductor laser chip 33 in accordance with a detection output of the power control light detecting means 37.
To provide a semiconductor laser in which the load on a semiconductor laser chip and the power consumption are reduced and that has a high output power that is necessary in recording information on an optical recording medium, it is necessary to efficiently utilize, for an output power control on the semiconductor laser, light that is emitted from the semiconductor laser chip.
FIG. 36 is a simplified system diagram showing the configuration of a still another conventional optical pickup apparatus 47. The optical pickup apparatus 47 is a conventional optical pickup apparatus that efficiently utilize, for an output power control, light that is emitted from a semiconductor laser chip. Since light that is emitted from the semiconductor laser chip 33 is diverging light, there exists light 48 that goes outward instead of entering the collimator lens 34. Part of the outgoing light 48 is detected by a power control light detecting means 37. An APC circuit 46 controls the output power of the semiconductor laser 32 to a constant value by supplying it with a signal for controlling a drive current of the semiconductor laser chip 33 in accordance with a detection output of the power control light detecting means 37.
However, such a still another conventional optical pickup apparatus 47 has the following problem. Since the power control light detecting means 37 utilizes light that is emitted from the semiconductor laser chip 33 and goes outward instead of entering the collimator lens 34, the quantity of light that can be detected by the power control light detecting means 37 is very small. That is, the detection light quantity is too small for the APC circuit 46 to control the output power of the semiconductor laser chip 33 with high accuracy; the reliability of the output power control is low.
FIG. 37 is a simplified side view showing the arrangement of a conventional optical pickup apparatus 51 using the front control method. The operation of the conventional optical pickup apparatus 51 will be described below. Light that is emitted from a semiconductor laser 52 is converted by a collimator lens 53 into generally parallel light, which enters a beam splitter 54. The beam splitter 54 is configured so as to transmit 95%, for example, of light that is emitted from the semiconductor laser 52 and received via the collimator lens 53 and to reflect the remaining light (5%). That is, 95% of the received light passes through the beam splitter 54 and goes toward an optical recording medium 61 and the remaining light (5%) is reflected by the beam splitter 54 toward a monitoring light detecting means 56.
The 5%-light reflected by the beam splitter 54 is condensed by a condenser lens 55 and shines on the monitoring light detecting means 56. The monitoring light detecting means 56 outputs, to the APC circuit 57, an electrical signal corresponding to a detection light quantity. The APC circuit 57 controls the power of output light of the semiconductor laser 52 to a constant value by supplying it with a drive current of the semiconductor laser 52 in accordance with the electrical signal supplied from the monitoring light detecting means 56.
On the other hand, the 95%-light that has passed through the beam splitter 54 is polarized by a quarter-wave plate 58 in passing through it, directed by an upward-directing mirror 59 toward the optical recording medium 61, and focused on the information recording surface of the optical recording medium 61 by an objective lens 60 to as to form a light beam spot having a prescribed size. The light beams focused on the information recording surface of the optical recording medium 61 is reflected by the optical recording medium 61, again passes through the objective lens 60, is reflected by the upward-directing mirror 59, passes through the quarter-wave plate 58, and enters the beam splitter 54. The beam splitter 54 is configured so as to reflect almost 100% of the light reflected from the optical recording medium 61. The light reflected from the beam splitter 54 is focused by a focusing lens 62, given astigmatism by a cylindrical lens 63, and applied to a light detecting means 64. A tracking error signal and a focusing error signal for operation controls and an information signal (RF signal) are generated on the basis of the reflection light coming from the optical recording medium 61 that is detected by the light detecting means 64.
For recording or reproduction of information by an optical pickup apparatus (particularly for the information recording), a high optical output power is needed to attain high accuracy and quality. Further, to reduce the load on the semiconductor laser and the power consumption, it is necessary to efficiently utilize the power of output light of the semiconductor laser. However, 5% of the quantity of light that is emitted in the front direction for information recording or reproduction is used for the output power control. As such, the conventional optical pickup apparatus 51 has a problem that the efficiency of utilization of light is low.
To solve the above problem, a conventional optical pickup apparatus is available in which light that is emitted from a semiconductor laser is detected directly (front monitoring). FIG. 38 is a simplified side view showing the arrangement of another conventional optical pickup apparatus 65 using the front monitoring method. The conventional optical pickup apparatus 65 is similar to the above conventional optical pickup apparatus 51, and hence components of the optical pickup apparatus 65 having corresponding components in the optical pickup apparatus 51 are given the same reference numerals as the latter and will not be described.
In another conventional optical pickup apparatus 65, light 66 that is emitted from the semiconductor laser 52 and goes off the optical axis without passing through the collimator lens 53, that is, the light 66 that is not used for information recording or reproduction, is detected directly by a monitoring light detecting means 56. The monitoring light detecting means 56 outputs, to an APC circuit 57, an electrical signal corresponding to a detection light quantity. The APC circuit 57 controls the output power of the semiconductor laser 52 to a constant value in accordance with the electrical signal supplied from the monitoring light detecting means 56. In the optical pickup apparatus 65, no part of light that is emitted from the semiconductor laser 52 for information recording or reproduction is divided and used for the output power control and hence the efficiency of utilization of the light emitted from the semiconductor laser 52 can be made high.
However, in the conventional optical pickup apparatus 65, since the monitoring light detecting means 56 directly detects only a very small part of light that diverges outside the outer periphery of the collimator lens 53, the detection light quantity is small and the electrical signal that can be used for controlling the output power of the semiconductor laser 52 is very weak. Therefore, the output power control on the semiconductor laser 52 by the APC circuit 57 cannot be performed with sufficient accuracy or stability, raising a problem that the reliability of the output power control is low.